Integrated circuit devices and other types of electronic components, such as componant 7 in FIG. 1A having multiple pinouts or leads 5 are often mounted onto the surface of a printed wire board 14 such that each pin or lead 5 of the electronic component 7 is mounted or soldered to a typically rectangularly-shaped deposit of copper (i.e. “pad12”) present on the surface of the printed wire board 14. (The means for mounting electronic components directly onto the surface or a printed wiring board is commonly referred to as “surface mount technology”.) The many component pads 12 which typically exist on the surface of the printed wiring board 14 are interconnected together in a predetermined configuration by thin copper signal lines (i.e. “traces10”). Conventionally, as is shown in FIG. 1, the practice in the industry has been to lay out a signal trace 10 so that it intersects one of the sides of the pad 12 at approximately the middle thereof. Typically too, the layout practice dictates that the trace 10 be orientated to intersect the side of the pad at a substantially ninety degree angle as shown.
As Referring to FIG. 1B, as the internal operating speed of integrated circuits (IC's) such a microprocessors and memories increase with improving semiconductor process technology, the digital signals 17 which travel along the printed circuit board 14 between the various components 7 thereof also have to reach their destinations 7a, i.e. trigger their input receivers 19, more quickly in order to take advantage of the increased IC operating speed. For example, as shown in FIG. 1c, in order to decrease memory access time in a computer, the control signals 42 produced by a memory controller 40 must reach the memory 64 more quickly and at a greater frequency.
FIG. 2A shows a typical digital binary signal 15 which goes from a low, or binary 0, state to a high, or binary 1, state. Since the transition of the signal from the low state to the high state or vice versa cannot be accomplished instantaneously, there exists a rise time tr and a fall time tf which respectively results in a sloping leading edge 16 and a sloping trailing edge 18. In order to make a control signal reach or trigger an input receiver faster, it is necessary to (a) minimize the rise and/or fall time, and (b) increase the frequency at which control signals are sent. In the present art, the rise/fall time has fallen to below one nanosecond, with signal frequency at 66 MHz.
FIG. 2B shows the binary digital signal 15 represented in the frequency domain. It will be noted that each frequency component i of the digital signal has a voltage v0(i) associated therewith (phase information is not shown in FIG. 2B).
On a printed circuit board, the junction between a signal trace and a component pad, such as the prior art interface shown in FIG. 1, represents an impedance discontinuity since (a) the width of the conducting path suddenly drastically increases at the pad and (b) the cross-sectional area or thickness of the conducting path increases due to solder present on the pad. When a digital signal such as that shown in FIGS. 2A and 2B travels across an impedance discontinuity, the voltage or power of the signal is split at the trace/pad junction so that a portion v0(i)/x1(i) of each frequency component i of the signal travels back in the opposite direction of the wave front. The remaining portion v0(i)/x2(i) of each frequency component i of the signal travels in the original or forward direction. The values x1(i) and x2(i) are greater than or equal to one. The greater the ratio of the larger impedance to the smaller impedance at the discontinuity, the more the voltage of the signal components will be affected by the discontinuity, i.e., the smaller the value of x1(i).
The portions v0(i)/x1(i) of each frequency component of the signal which travel in the opposite direction will similarly reflect at the next impedance discontinuity in their path, and the same will happen to the frequency components v0(i)/x2(i) which travel in the forward direction. The reflected portions of the signal will recombine with the wavefront and modify its appearance. In the frequency domain, this will be visible as a change in voltage associated with each frequency component. In the time domain, this will appear as one or more “glitches” or “inflection points”. An example of one such glitch caused by a single discontinuity is shown in idealized form in the time domain diagram of FIG. 3 where at time/position t0 the forward portion Σv0(i)/x2(i) of the signal travels across the impedance discontinuity presented by the prior art trace/pad junction and a short time t1 later the reflected and rebounded portion Σv0(i)/x1(i) of the signal recombines.
The physical size of electronic components mounted on a printed circuit board means that the components have to be distributed across the board in such a way that relatively long signal traces cannot be avoided. Inflection points or glitches on the signal will be produced by the impedance discontinuities along the conduction path the signal travels. These non-monotonic wave forms appear as multiple rising (or falling) edges where only one rising (or falling) edge was desired. One or a combination of the glitches or inflection points may produce false triggers at input receivers. The invention seeks to reduce the tendency of this phenomenon.